Given stp conditions, how many milliliters of hydrogen gas are produced by the reaction 0.020 mol of magnesium with excess of hydrochloric acid?

The balanced equation for the reaction of magnesium and hydrochloric acid is,
                             Mg + 2HCl ---> H2 + MgCl2
From the reaction, every 0.02 moles of magnesium will produce 0.02 moles H2. Using the ideal gas equation,
                             V = nRT / P
Substituting the known values,
                            V = (0.02 mol)(0.0821 L.atm / mol K)(273 K) / 1 atm 
                               = 0.45 L = 450 mL
Therefore, the hydrogen gas produced is approximately 450 mL.